1. Field of the Invention
The present invention relates generally to an apparatus and method for performing a access network selection in a wireless communication system, and in particular, to an apparatus and method for selectively accessing a wireless communication network in a multi-wireless communication network including a plurality of wireless communication systems.
2. Description of the Related Art
In general, cellular systems, typical wireless communication systems, are classified into a synchronous CDMA (Code Division Multiple Access) system and an asynchronous UMTS (Universal Mobile Telecommunications System) system. Such cellular systems have developed from an early system mainly providing a voice communication service into an improved high-speed data system and a system providing a data service and a voice service together at high speed. In addition, a GPS (Global Positioning System) system and a wireless LAN (Local Area Network) system are currently being used, and it is expected that more diversified systems will appear in the future.
Particularly, for a cellular network, active research has been carried out on hierarchical structure-based systems in order to provide various types of services and increase network efficiency. A hierarchical structure-based cellular network diversifies coverage of cells and controls the diversified cell coverage on a hierarchical basis. Until recently, research has been mainly conducted on a hierarchical structure-based system for a single network.
FIG. 1 is a diagram illustrating a method for providing a currently available service by a cellular system and a satellite system. As illustrated in FIG. 1, a cellular system has manifold cells and groups cells being similar in coverage into one layer. That is, FIG. 1 illustrates a cellular system having Pico cells 120 or a cellular system having microcells 130. Alternatively, the Pico cells and the microcells may coexist in a single system. That is, cells having different sizes may exist in one system. In addition, a wide coverage 140 can be defined as a communication area covered by a satellite 110 without discrimination of cells. Generally, however, one network groups cells having similar coverage in size into one layer.
Accordingly, such a hierarchical structure-based cellular system has various proposed methods for access network selection by appropriately selecting a higher layer cell or a lower layer cell according to a characteristic of a user. A conventional selection method includes a method for selecting a call connection layer according to a moving speed of a user. This method selects a cell layer according to a traveling velocity of the user. For example, when a moving speed of a user does not exceed a predetermined threshold, a call is assigned to a cell having a small coverage such as a microcell or a picocell. However, when the moving speed of a user exceeds the predetermined threshold, a call is assigned to a cell having a large coverage such as a microcell to prevent frequent occurrence of handoff and deterioration of service quality due to the frequent handoff. This method has the advantage of reducing frequency of the occurrence of handoff by utilizing a hierarchical cell structure, thereby contributing to efficient utilization of network resources.
Another access network selection method in the hierarchical structure-based cellular system includes a selection method considering inter-layer traffic distribution. When a mobile station is located in a cell-overlapping region (or handoff region), the selection method chooses a best cell layer from several cell layers for performance improvement and stable service of a network considering inter-layer traffic distribution, and compulsorily performs a handoff, when necessary. This method is advantageous in that network efficiency and service quality can be simultaneously considered.
However, the above-stated methods are all characterized in that cell selection for call connection is performed by a network rather than a user. In a next generation mobile communication network, various types of wireless access networks coexist, and the type and quality of services provided will be more diversified. Therefore, when the existing network-centered access network selection method is used intently, there is high probability that a call control procedure will become very complicated. In addition, the network-centered selection method cannot sufficiently reflect different personal requests of the users. Furthermore, when different wireless systems coexist, compatibility between them is not secured.